It seems beyond coincidence for it to have emerged simultaneously from two great thinkers at about the same time. Conditions must have been ripe for its emergence. But why?
The fact that the geometry of physics had become so difficult to deal with in the late 17th century prompted people to look around for alternatives, and when they couldn't find them, invent them themselves. So you have Newton and Leibniz inventing calculus at pretty much exactly the same time independently of each other, because they're both trying to find a solution to the same problem.